Know Before You Go... Final Fantasy XV

There have been a couple demos that have come out you could give a try and see how you like the gameplay/mechanics before you buy.
Full disclosure: I am literally pre-downloading the digital deluxe edition at this very moment because I'm a huge FF fanboy and own at least one version of every main series game, but in most cases multiple copies, plus a huge library of spin offs and FF branded games, so I basically have to buy every new main game when they come out. So you know, try it out first before committing to a life long addiction to the teat of Square-Enix.

Did you feel the AH video spoiled much, if anything if you've already played both demos and watched the movie and anime mini-series? I've been avoiding that video because want to genuinely experience anything new in-game, since the release is just a few days off now. But if the first hour is just exposition of events that had already been covered in other media, and a rehash of the tutorials from the demos, then there's nothing spoilery in it for me, and I'd like to watch it now, but I'm so conflicted without knowing either way. :(

If you haven't watched any of the people's "First few hours" gameplay type videos then there is some spoilers so I would avoid it till next week. If you do want to watch it without spoiling anything at all then I would skip the video till around 39:30 minutes where it is just them goofing about with a hunt/bounty mission.

Thanks Zikoro, I'll just watch it in the wee hours of Tuesday morning as I try to fall asleep after spending all night playing FF VX. Already pre-downloaded and ready to go! 

If you want to get into it, the Platinum Demo is free, but I'd recommend playing it after watching Brotherhood. More on it below. If you want a full game, I'd recommend Type-0. It was originally a mobile game so the mechanics aren't complicated, but it features most of the series tropes, despite all the characters being teenagers and constant tool tips. 7 and 10 are also available on PS4 and everything but 12 is available on PC. 7 and 10 are among the highest-rated. 

If you don't feel like playing, there's several movies and anime.

Unlimited is 2 seasons, and introduces a multiverse that loosely ties the games' universes together, while not featuring any specific one.

As mentioned, Kingsglaive is a CGI film tying into 15,

and Brotherhood introduces the four main characters of 15.

The Spirits Within is a near-future sci-fi CGI movie taking place on Earth and not tying into any games. Honestly not a great story, but the animation is great for 2001. You can find it at just about any used media store for like $2.

FF7 is the fan favorite, and it got a 20-minute anime called Last Order. showing the backstory, but as it's revealed over the course of the game through various twists, I'd say to wait until you finish the game. Episode Denzel shows what a character new to the movie (more on that) was doing during the game. Another 20-minute OVA. Advent Children is a full-length CGI film a la Kingsglaive and Spirits Within taking place 2 years after FF7.

Legend of the Crystals (Sometimes just called Final Fantasy) is an anime that takes place a few centuries after FF5 but doesn't rely very heavily on it. I played the first 20 minutes of FF5 and understood the anime perfectly.

Buuuut if you want to start with 15 that's fine. I'd say just get Platinum Demo and watch Brotherhood for free (Officially) on Youtube.